Methanol+Ethylene Glycol (MEG)
Properties keywords: liquid, water soluble, non-linear density behaviour, toxic
Analogue keywords: non-linear density currents, pyroclastic flows
Common names: methanol + ethylene glycol, Ch4O + HO-CH2CH2-OH
General Information: MEG is a combination of two liquids: methanol, a toxic, low viscosity liquid alcohol used for insecticide, paint stripper, antifreeze and cleaning fluids; and ethylene glycol, an odourless, colourless, toxic liquid of low volatility and viscosity, used for antifreeze and in synthetic rubbers and adhesives.
Note: Ethylene glycol on its own is sometimes also referred to as MEG.
Properties
The major property of MEG for use in analogue experiments is its non-linear density behaviour when mixed with water (e.g. Huppert et al., 1986). The density of a MEG-water mixture varies quadratically with the amount of water in the solution and is initially less dense than water (Figure 1). It will therefore sink in water with mixing and then rise as it becomes more diluted by the water.
Figure 1. Density behaviour of MEG-water mixtures, from Woods and Caulfield, 1992).
Properties of pure methanol and pure ethylene glycol, respectively, are as follows: density = 792 kg m-3 and 1113 kg m-3, viscosity = 0.587 Pas and 0.021 Pas (at 20°C), melting point = -97.6°C and -13.4°C, boiling point = 64.7°C and 197.0°C, refractive index = 1.33 and 1.43 and surface tension (at 25°C) is 0.023 N/m and 0.048 N/m (Tsierkezos and Molinou, 1999; Dow Chemical Company). The mixture is soluble in water and other organic solvents.
Applications
As a result of MEG-water mixtures acting as non-linear density currents, they can be useful for simulating the behaviour of pyroclastic flows, whereby a hot dense current of ash and air flowing down a slope mixes with air and becomes buoyant as it entrains the less dense surrounding air. It has therefore been used in analogue experiments to study the effects of buoyancy reversal on mixing and the transition from a plume to a pyroclastic flow (e.g. Turner 1966; Woods and Caulfield, 1992; Woods, 2010). It is thought to be more analogous to nature than salt and fresh water density currents because of the greater effect of buoyancy.
However, the buoyancy reversal in MEG-water is opposite to that of a pyroclastic flow in air and therefore experiments have typically been performed upside down, where the roof of the experimental apparatus represents the ground, underneath which the mixture flows upwards, and images are subsequently inverted to simulate the behaviour of pyroclastic flows in nature (Huppert et al., 1986; Woods and Caulfield, 1992; Woods and Bursik, 1994).
Limitations and tips for use
Below are several significant limitations to using MEG as an analogue for pyroclastic density currents, namely because it is not analogous to the multiphase gas-particle mixtures we now recognise PDCs to be (Sulpizio et al., 2014).
1) Because PDCs may be controlled by particle interactions an experiment controlled by buoyancy and turbulent flow is not very relevant, 2) there is a much greater density contrast between a pyroclastic flow and air than MEG and water, resulting in different buoyancy forces, 3) MEG is fairly homogenous and does not show the layer stratification observed in PDCs, and 4) MEG-water currents cannot account for sedimentation processes and therefore may only be valid for fine-grained plumes where the solid material remains suspended in the plume (Woods and Caulfield, 1992).
Methanol and ethylene glycol are both toxic materials if ingested and should not be allowed to come into contact with skin, eyes or clothing. It should not be inhaled.
References
Dow Chemical Company MSDS. Accessed January 2016: http://www.dow.com/ethyleneglycol/about/properties.htm
Huppert HE, Turner JS, Carey SN, Sparks RSJ, and Hallworth MA (1986) A laboratory simulation of pyroclastic flows down slopes. Journal of Volcanology and Geothermal Research 30: 179-199
Sulpizio R, Dellino P, Doronzo DM, and Sarocchi D (2014) Pyroclastic density currents: state of the art and perspectives. Journal of Volcanology and Geothermal Research 283: 36-65
Tsierkezos NG and Molinou IE (1999) Densities and viscosities of ethylene glycol binary mixtures at 293.15K. Journal of Chemical Engineering Data 44: 955-958
Turner JS (1966) Jets and plumes with negative or reversing buoyancy. Journal of Fluid Mechanics 26: 779-792
Woods AW (2010) Turbulent plumes in nature. Annual Review of Fluid Mechanics 42: 291-412
Woods AW and Bursik MI (1994) A laboratory study of ash flows. Journal of Geophysical Research 99, B3: 4375-494
Woods AW and Caulfield CP (1992) A laboratory study of explosive volcanic plumes. Journal of Geophysical Research 97, B5: 6699-6712